libata: don't configure downstream links faster than the upstream link

There's nothing to be gained by configuring downstream links faster
than the upstream link and such configurations cause problems on
certain PMPs.  Limit downstream link speed by the upstream link speed.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>